Field of the Invention
The present invention relates to an abnormality diagnostic device for a particulate filter which is arranged in an exhaust passage of an internal combustion engine for trapping PM (Particulate Matter) in exhaust gas.
Description of the Related Art
In the past, there has been known a technology in which a particulate filter (hereinafter also simply referred to as “a filter”) for trapping PM in exhaust gas is arranged in an exhaust passage of an internal combustion engine. A failure such as melting loss, damage, etc., may occur in the filter. If such a failure of the filter occurs, an amount of PM, which flows out from the filter without being trapped thereby, will increase. When an abnormality occurs in the filter, such as a failure of the filter, removal of the filter from the exhaust passage, etc., an increase of PM to be released into the atmosphere will be caused. Accordingly, there has been developed a technology in which a PM sensor is arranged in the exhaust passage at the downstream side of the filter, so that an abnormality diagnosis of the filter is carried out based on an output value of the PM sensor. As the PM sensor used for such an abnormality diagnosis of the filter, there has been known one which has a pair of electrodes as a sensor element, and which outputs a signal corresponding to an amount of the PM deposited in between the electrodes.
In addition, in Japanese patent laid-open publication No. 2011-179467, there is disclosed a technology in which the presence or absence of a failure of a filter is determined by making a comparison between an output value of a PM sensor, which is arranged in an exhaust passage at the downstream side of the filter, and an estimated value of an amount of PM deposition in the PM sensor. With the technology described in this reference, an amount of PM flowing out from the filter is estimated when assuming that the filter is in a predetermined state. Then, the estimated value of the amount of PM deposition in the PM sensor is calculated based on an integrated value of the amount of outflowing PM. By making a comparison between the estimated value of the amount of PM deposition thus calculated and an actual output value of the PM sensor, it becomes possible to grasp the state or condition of the filter.
Moreover, in Japanese patent laid-open publication No. 2012-122399, there is also disclosed a technology in which failure diagnosis of a filter is made based on the timing at which electrical energization or conduction is started due to the deposition of PM between the electrodes of a PM sensor which is arranged in an exhaust passage at the downstream side of the filter. With this technology described in this reference, in cases where the timing at which electrical conduction or energization between the electrodes of the PM sensor has started earlier than or before a start timing of electrical conduction or energization where it is assumed that the filter has failed, a determination is made that the filter is in failure.
Further, in Japanese patent laid-open publication No. 2012-077716, there is disclosed a technology with respect to the detection of an abnormality in a PM sensor. With the technology described in this reference, the PM deposited in between electrodes of the PM sensor is combusted and removed by being heated by means of a heater. Then, an abnormality in the PM sensor is detected based on a change in the resistance value between the electrodes with respect to a combustion removal period of time.